The present invention relates to a modulator-demodulator (modem) for video data.
A modem is commonly used to transform digital computer data into an analog form suitable for transmission over a voice grade telephone network. A carrier signal within the bandwidth of the telephone network is modulated with the digital data by one of a variety of methods. Many inexpensive modems in use today use either phase-shift keyed (PSK) modulation or frequency-shift keyed (FSK) modulation. These modems can be implemented digitally because the computer data is contained in the zero crossings of the modulated carrier signal. Thus, digital circuitry which counts and quantifies the zero crossings can be used for the demodulator. More expensive modems which use other types of modulation schemes (such as multi-level quadrature amplitude modulation (QAM)) typically require complicated and expensive filters and other components.
The basic unit of information transmitted by a modem is called a baud or a symbol. Each baud or symbol is a packet of information containing at least one binary bit. In higher bit rate modems, however, the number of bits contained in the baud or symbol is typically usually greater than one. For example, the Bell 212 modem transmits information at six hundred baud or symbols per second, but each baud or symbol contains enough information to represent two bits. Therefore, the data rate is twelve hundred bits per second.
The usable voice grade telephone network bandwidth is approximately 3400 Hz. Because of the distances involved and the varying condition and age of the components, the telephone network has various undesirable characteristics which can cause data errors during use of the telephone network as a communication link. Group delay, which is the variation in transmission time depending upon the frequency of the signal transmitted, varies from approximately 1 to 3 milliseconds across the usable bandwidth of approximately 3400 Hz. There are also frequency shifts of up to approximately 10 Hz in both positive and negative directions and other impairments, such as phase and amplitude hits, burst noise and phase jitter. All but one of these degradations results in jitter in the recovered data (amplitude hits or dropouts do not have this effect). This resultant jitter leads to inaccuracies in the recovered data clock and subsequent errors in received data. Group delay leads to another effect called inter-symbol interference (ISI) by causing amplitude information to be spread across symbol intervals. This means that the amplitude value of adjacent symbols is distorted.
For the higher data rate modems, simple PSK, FSK or QAM modulation systems are not sufficient, and multilevel schemes or vestigal side band or single side band techniques must be used. These systems become quite expensive, requiring precision narrow-band filters, sensitive phase detectors, analog multipliers and other components. In addition, as the amount of information increases, the decision threshold for symbol decoding decreases (e.g., less distance between amplitude levels). Thus, the effect of ISI becomes more and more important. Virtually all high-speed modems use some form of adaptive equalization to reduce the effects of group delay and hence ISI. However, adaptive equalization is expensive and complicated to implement.
For video data, the most familiar modulation scheme is that used for television transmission. In standard television transmission, the TV picture is composed of an array of dots or pixels. For black and white television, the pixels range in values from black through intermediate shades of gray to white. In order to transmit the TV picture, a high-frequency (MHz range) carrier signal typically has its amplitude modulated in accordance with the brightness or darkness of the individual pixels. This amplitude level is decoded by a demodulator in the television set which controls the intensity of the voltage driving the electron gun that impacts upon the phosphor at each pixel position on the television tube. This analog modulation system requires a high-frequency bandwidth because of the abrupt changes in amplitude, especially in changes from black to white. These abrupt amplitude changes, when used to modulate the carrier, introduce high-frequency components into the signal, thus requiring a large bandwidth for transmission.
For digital video or facsimile transmission, the brightness of each pixel is transformed into a digital value. Typically, 8-10 bits are used for video (i.e., 256 to 1024 values) and one bit is used for facsimile (black and white). One specific example would be where sixteen values are used representing a range of brightness from white through fourteen intermediate shades of gray to black. These sixteen values require a minimum of four bits to represent each value. The digital bit stream can then be supplied to a modem which is used for digital data.
The hostile environment of the telephone network makes it difficult to use higher data rates for video data without experiencing severe degradation of picture quality due to errors. For instance, the intersymbol interference of the telephone network can have the effect of changing one bit of each four-bit word representing a pixel. This change of one bit value can result in a large change in the decoded value of the pixel, such as a change from black to white. Such an error would cause white dots to appear in an area of the picture which should be black, or vice-versa.